To reduce a burden on the environment, exhaust gas purification catalysts in which a noble metal such as platinum (Pt) is supported by a metal oxide support such as aluminum oxide (Al2O3) have been widely used for removing harmful substances contained in exhaust gas of internal combustion engines of vehicles such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx).
In recent years, emission of non-methane organic gas (NMOG) has been strictly regulated among hydrocarbons (HC).
To meet the regulation, an exhaust gas purification device is generally installed in an exhaust system of an internal combustion engine for removing harmful substances in the exhaust gas.
For exhaust gas purification devices, an exhaust gas purification catalyst using a noble metal is typically used. However, since noble metals are expensive, it has been required to reduce the usage of noble metals in terms of cost reduction.
Among noble metals, it has been particularly required to reduce the usage of rhodium (Rh), since rhodium (Rh) is particularly expensive, although it has high purification activity on nitrogen oxides (NOx).
Further, it has been known that exhaust gas purification catalysts that contain aluminum oxide (Al2O3) or a cerium (Ce)-containing oxide as a main component of a catalyst support degrade with age. For example, rhodium (Rh) forms solid solution with aluminum oxide (Al2O3), or rhodium (Rh) is covered by the cerium (Ce)-containing oxide.
Since such degradation results in a degradation of the purification performance of the exhaust gas purification catalysts against nitrogen oxides (NOx), it has been required to preliminarily increase the usage of rhodium (Rh) in order to cover the degradation of the performance.
To cope with the problem, a hydrocarbon reforming catalyst has been proposed in which rhodium (Rh) is supported by a complex oxide containing lanthanum (La) and zirconium (Zr) (see Patent Document 1).